Electrochemistry can be summarized as two simple and mirrored concepts: (1) the passage of an electrical current to cause chemical transformation of matter; and (2) the chemical transformation of matter to cause the passage of an electrical current.
Electrochemistry involves numerous oxidation and reduction reactions at or near anodes, cathodes and electrolytes in various electrochemical and electrolytic cells. The specific reactions can occur by applying a current to various chemical reactants contained in a system to achieve a desired chemical species (e.g., electrolysis) or conversely, chemical reactions can result in the production of electricity (e.g., electrochemical reactions). In certain systems both electrolytic and electrochemical reactions occur (e.g., rechargeable cells or rechargeable batteries). Oxidation reactions are those reactions which lose electrons, whereas reduction reactions are those reactions which gain electrons. Oxidation and reduction reactions occur in many chemical systems. For example, the rusting of metals, photosynthesis in the leaves of green plants, the conversion of foods to energy in the body, the generation of current from batteries and fuel cells, etc., are all examples of chemical changes that involve the transfer of electrons from one chemical species to another. When such reactions result in electrons flowing through a wire or when the flow of electrons causes a particular reduction reaction to occur, the processes are generally referred to as electrochemical reactions. Whereas the study and/or application of these electrochemical reactions is often referred to as electrochemistry.
The applications of electrochemistry are widespread, as discussed above. Additionally, electrical measurements are used to monitor chemical reactions in a wide variety of reactions, including those reactions occurring in an element as small as a living cell. Further, numerous important chemicals are manufactured by electrochemical means. In addition, electrochemistry is used to form a variety of important metals (e.g., aluminum and magnesium). Still further, electrochemistry is used in corrosion protection, brine electrolysis, electrocrystallization, electroforming, electrolyte pickling, electrometallurgy, electroplating, electrorefining, electrowinning, galvanizing, photoelectrochemistry; photoelectrosynthesis and sonoelectrochemistry.
Many reactions that occur in the aforementioned systems appear to involve a shifting of electron density from one atom to another. Collectively, this shifting of electron density is referred to as oxidation/reduction reactions or more simply, redox reactions. The term “oxidation” refers to the loss of electrons by one reactant and the term “reduction” refers to the gain of electrons by another. Oxidation and reduction typically occur together. In particular, if one substance is oxidized, another substance is typically reduced. Otherwise, electrons would be a product of a reaction, and this has not been observed in these reactions. During a redox reaction the substance that accepts the electrons that another substance loses is known as an “oxidizing agent”, because it helps something else to be oxidized. Moreover, the substance that supplies electrons is termed the “reducing agent” because it helps something else to be reduced.
The various reactions that occur in the aforementioned electrochemical systems are driven by energy. The energy comes primarily in two different forms: chemical and electrical. There are many other forms of energy that drive chemical transformation, however, including thermal, mechanical, acoustic, and electromagnetic. Various features of each type of energy are thought to contribute in different ways to the driving of chemical reactions. Irrespective of the type of energy involved, chemical reactions and transformations are undeniably and inextricably intertwined with the transfer and combination of energy. An understanding of energy is, therefore, vital to an understanding of chemical reactions, including electrochemical reactions.
A chemical reaction can be controlled and/or directed either by the addition of energy to the reaction medium in the form of thermal, mechanical, acoustic, electrical, magnetic, and/or electromagnetic energy or by means of transferring energy through a physical catalyst. These methods are traditionally not that energy efficient and can produce, for example, either unwanted by-products, decomposition of required transients, and/or intermediates and/or activated complexes and/or insufficient quantities of preferred products of a reaction.
It has been generally believed that chemical reactions occur as a result of collisions between reacting molecules. In terms of the collision theory of chemical kinetics, it has been expected that the rate of a reaction is directly proportional to the number of the molecular collisions per second:rate α number of collisions/sec
This simple relationship has been used to explain the dependence of reaction rates on concentration. Additionally, with few exceptions, reaction rates have been believed to increase with increasing temperature because of increased collisions.
The dependence of the rate constant k of a reaction can be expressed by the following equation, known as the Arrhenius equation:k=Ae−Ea/RT where Eα is the activation energy of the reaction which is the minimum amount of energy required to initiate a chemical reaction, R is the gas constant, T is the absolute temperature and e is the base of the natural logarithm scale. The quantity A represents the collision rate and shows that the rate constant is directly proportional to A and, therefore, to the collision rate. Furthermore, because of the minus sign associated with the exponent Eα/RT, the rate constant decreases with increasing activation energy and increases with increasing temperature.
Normally, only a small fraction of the colliding molecules, typically the fastest-moving ones, have enough kinetic energy to exceed the activation energy, therefore, the increase in the rate constant k has been explained with the temperature increase. Since more high-energy molecules are present at a higher temperature, the rate of product formation is also greater at the higher temperature. But, with increased temperatures there are a number of problems which can be introduced into the cell reaction system. With thermal excitation other competing processes, such as bond rupture, may occur before the desired energy state can be reached. Also, there are a number of decomposition products which often produce fragments that are extremely reactive, but they can be so short-lived because of their thermodynamic instability, that a preferred reaction may be dampened.
In electrochemical reactions, it is generally believed that chemical transformations occur as the result of the passage of an electrical current, or conversely that a chemical reaction produces an electrical current. In this regard, electrochemistry involves the science of ionically conducting solutions, as well as the science of electrically charged interfaces. There are four important aspects of ionically conducting solutions: (1) ion interactions with the solvent; (2) ion interactions with other ions (either of the same or different species); (3) movement of ions in solutions; and (4) ionic liquids or “pure electrolytes”
Electrodics, the same science of electrically charged interfaces, involves the transfer of charge across solid-solution interfaces and interfacial regions. Electrodics thus involves the transfer of electrical charge between two phases of matter. These phases can exist on surfaces of materials not customarily thought of as solid-solution interfaces. Corrosion is a good example of this, wherein a material is covered by a thick invisible film of moisture. Atoms on the surface of the material leave the material and dissolve into the ion-containing film of moisture. Thus, in electrochemistry, the familiar kinetic catalysts of chemical reactions by molecules and atoms colliding with each other is replaced by species colliding with electrodes. In this regard, the electrodes can be thought of as separate “charge-transfer catalysts”.
Radiant or light energy is another form of energy that may be added to the reaction medium that also may have negative side effects but which may be different from (or the same as) those side effects from thermal energy. Addition of radiant energy to a system produces electronically excited molecules that are capable of undergoing chemical reactions.
A molecule in which all the electrons are in stable orbitals is said to be in the ground electronic state. These orbitals may be either bonding or non-bonding. If a photon of the proper energy collides with the molecule the photon may be absorbed and one of the electrons may be promoted to an unoccupied orbital of higher energy. Electronic excitation results in spatial redistribution of the valence electrons with concomitant changes in internuclear configurations. Since chemical reactions are controlled to a great extent by these factors, an electronically excited molecule undergoes a chemical reaction that may be distinctly different from those of its ground-state counterpart.
The energy of a photon is defined in terms of its frequency or wavelength,E=hv=hc/λwhere E is energy; h is Plank's constant, 6.6×10−34 J·sec; v is the frequency of the radiation, sec−1; c is the speed of light; and λ is the wavelength of the radiation. When a photon is absorbed, all of its energy is typically imparted to the absorbing species. The primary act following absorption depends on the wavelength of the incident light. Photochemistry studies photons whose energies lie in the ultraviolet region (e.g., 100 Å-4000 Å) and in the visible region (e.g., 4000 Å-7000 Å) of the electromagnetic spectrum. Such photons are primarily a cause of electronically excited molecules.
Since the molecules are imbued with electronic energy upon absorption of light, reactions occur from different potential-energy surfaces from those encountered in thermally excited systems. However, there are several drawbacks of using the known techniques of photochemistry, that being, utilizing a broad band of frequencies thereby causing unwanted side reactions, undue experimentation, and poor quantum yield. Some good examples of photochemistry are shown in the following patents.
In particular, U.S. Pat. No. 5,174,877 issued to Cooper, et al. al., (1992) discloses an apparatus for the photocatalytic treatment of liquids. In particular, it is disclosed that ultraviolet light irradiates the surface of a prepared slurry to activate the photocatalytic properties of the particles contained in the slurry. The transparency of the slurry affects, for example, absorption of radiation. Moreover, discussions of different frequencies suitable for achieving desirable photocatalytic activity are disclosed.
Further, U.S. Pat. No. 4,755,269 issued to Brumer, et al. al., (1998) discloses a photodisassociation process for disassociating various molecules in a known energy level. In particular, it is disclosed that different disassociation pathways are possible and the different pathways can be followed due to selecting different frequencies of certain electromagnetic radiation. It is further disclosed that the amplitude of electromagnetic radiation applied corresponds to amounts of product produced.
Selective excitation of different species is shown in the following three (3) patents. Specifically, U.S. Pat. No. 4,012,301 to Rich, et al. al., (1977) discloses vapor phase chemical reactions that are selectively excited by using vibrational modes corresponding to the continuously flowing reactant species. Particularly, a continuous wave laser emits radiation that is absorbed by the vibrational mode of the reactant species.
U.S. Pat. No. 5,215,634 issued to Wan, et al., (1993) discloses a process of selectively converting methane to a desired oxygenate. In particular, methane is irradiated in the presence of a catalyst with pulsed microwave radiation to convert reactants to desirable products. The physical catalyst disclosed comprises nickel and the microwave radiation is applied in the range of about 1.5 to 3.0 GHz.
U.S. Pat. No. 5,015,349 issued to Suib, et al. al., (1991) discloses a method for cracking a hydrocarbon to create cracked reaction products. It is disclosed that a stream of hydrocarbon is exposed to a microwave energy which creates a low power density microwave discharge plasma, wherein the microwave energy is adjusted to achieve desired results. A particular frequency desired of microwave energy is disclosed as being 2.45 GHz.
Photoelectrochemistry has focussed on three main areas. In the first, light is shone on a metal electrode, but metals absorb broadband light very poorly and so this method is not greatly effective. The second area of photoelectric chemistry involves the passage of electrons in a solution by absorption of light by photoactive species resulting in electron generation. Again, these techniques use broadband methods and have drawbacks. The third area of photoelectricalchemistry considered by many to be the most promising involves the absorption of light by semiconductors in cell reaction systems, elevating electrons from valence bands to conduction bands. The semiconductors in electrodes function as photo-induced charge transfer catalysts.
Physical catalysts are well known in the art. Specifically, a physical catalyst is a substance which alters the reaction rate of a chemical reaction without appearing in the end product. It is known that some reactions can be speeded up or controlled by the presence of substances which themselves appear to remain unchanged after the reaction has ended. By increasing the velocity of a desired reaction relative to unwanted reactions, the formation of a desired product can be maximized compared with unwanted by-products. Often only a trace of physical catalyst is necessary to accelerate the reaction. Also, it has been observed that some substances, which if added in trace amounts, can slow down the rate of a reaction. This looks like the reverse of catalysis, and, in fact, substances which slow down a reaction rate have been called negative catalysts or poisons. Known physical catalysts go through a cycle in which they are used and regenerated so that they can be used again and again. A physical catalyst operates by providing another path for the reaction which can have a higher reaction rate or slower rate than available in the absence of the physical catalyst. At the end of the reaction, because the physical catalyst can be recovered, it appears the physical catalyst is not involved in the reaction. But, the physical catalyst must somehow take part in the reaction, or else the rate of the reaction would not change. The catalytic act has historically been represented by five essential steps originally postulated by Ostwald around the late 1800's:
1. Diffusion to the catalytic site (reactant);
2. Bond formation at the catalytic site (reactant);
3. Reaction of the catalyst-reactant complex;
4. Bond rupture at the catalytic site (product); and
5. Diffusion away from the catalytic site (product).
The exact mechanisms of catalytic actions are unknown in the art but it is known that physical catalysts can speed up a reaction that otherwise would take place too slowly to be practical.
There are a number of problems involved with known industrial catalysts: firstly, physical catalysts can not only lose their efficiency but also their selectivity, which can occur due to, for example, overheating or contamination of the catalyst; secondly, many physical catalysts include costly metals such as platinum or silver and have only a limited life span, some are difficult to rejuvenate, and the precious metals may not be easily reclaimed. There are numerous physical limitations associated with physical catalysts which render them less than ideal participants in many reactions.
Accordingly, what is needed is an understanding of the catalytic process so that biological processing, chemical processing, industrial processing, etc., can be engineered by more precisely controlling the multitude of reaction processes that currently exist, as well as developing completely new reaction pathways and/or reaction products. Examples of such understandings include methods to catalyze reactions without the drawbacks of: (1) known physical catalysts; and (2) utilizing energy with much greater specificity than the prior art teachings which utilize less than ideal thermal and electromagnetic radiation methods and which result in numerous inefficiencies.
Accordingly, what is also needed are techniques for electrochemistry which improve the functioning of electrodes as charge-transfer catalysts. Further, improvements are required in the transport of ions and charges through electrolyte solutions. Passage of electrons through metals and semiconductors is not rate-limiting, rather it is the passage of ions and charged species through solutions that is slow.